MAS-24 with Microporous

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CO2 Reforming of CH4 over MgO-Doped Ni/ MAS-24 with Microporous ZSM-5 structure Yinchuan Zhao, Bingsi Liu, and Roohul Amin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00935 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 10, 2016

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Industrial & Engineering Chemistry Research

CO2 Reforming of CH4 over MgO-Doped Ni/MAS-24 with Microporous ZSM-5 structure Yinchuan Zhao, Bingsi Liu,* Roohul Amin Department of Chemistry, School of Science, Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ABSTRACT: Mesoporous alumino-silicate with ZSM-5 structure was synthesized from ZSM-5 nano-clusters and used as support, which possessed strong acidity, large surface

areas

and

high

hydrothermal

stability.

A

series

of

0-12wt%MgO-10wt%Ni/MAS-24 were prepared via a sol-gel method and characterized by XRD, N2 adsorption, FT-IR, H2-TPR, CO2-TPD, UV-Vis, TG/DSC and HRTEM techniques. The effects of catalyst composition, reaction temperature on the catalytic performance in CO2/CH4 reforming were studied. The catalytic stability over 9wt%MgO-10Ni/MAS-24 for 60 h indicated that the conversion of CH4 and CO2 maintained 93% and 99%, respectively, the selectivity of H2 and CO for 99% at 800 o

C although the conversion of CH4 decreased to 83% in the later 30 h due to high

dispersion of Ni particles via synergistic action between MgO and NiO nanoparticles as well as basic effect on the surface, which prevented Ni0 active particles from aggregating and coke formation during CH4/CO2 reforming reaction. Keywords: MAS-24 support, ZSM-5 zeolite crystallites, XMgO-10Ni/MAS-24 catalyst, CO2/CH4 reforming, Synergistic action 1. INTRODUCTION

*

Corresponding author, Tel: +86 22 27892471; fax: +86 22 27403475. E-mail: [email protected] (B.S. Liu). 1

ACS Paragon Plus Environment


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With the excessive consumption of fossil fuels, the environmental pollution is becoming more and more serious, especially the problem of greenhouse effect derived from the greenhouse gases such as CO2 and CH4. In the last two decade, the effective use and transformation of these greenhouse gases have been the focus concern of researchers all over the world, including steam reforming, partial oxidation, and CO2 dry reforming of methane.1,2 The CO2 reforming reaction of CH4 (CH4 + CO2 = 2H2 + 2CO, ∆ H0 (298 K) = 247 kJ·mol-1) can not only eliminate greenhouse gases, but also produce valuable synthetic gas with H2/CO ratio close to 1 which can be used directly for F-T synthesis.3,4 For CH4/CO2 reforming reaction, Nickel-based catalysts have been widely studied due to their high activity and low cost in comparison to noble metals-based ones.5,6 However, they easily deactivate as a result of the formation of carbon deposition and the sintering of active metal Ni0 particles during the high temperature reaction, which is the main problem to prevent the industrialization of the nickel-based catalyst.3,7 Therefore, how to design and prepare catalysts performed high catalytic activity and stability naturally becomes the vital problem. In general, the performance of the nickel-based catalysts is influenced by the nature of support, the composition of active component and the catalyst preparation technique.8-10 It is well known that the mesoporous silica materials, such as MCM-41, SBA-15 and KIT-6 with large specific surface areas and big pore size are used extensively as the supports of catalysts for the CO2 reforming of methane. In the meantime, the corresponding Ni-supported catalysts exhibited the high dispersion of active metals and overcame the diffusion barrier for reactants and products, leading to

2


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relatively high performance in CH4/CO2 reforming reaction.2,11,12 However, these pure silica materials have low hydrothermal stability, which is unfavorable to prevent from the sintering of active species due to the high temperature and the reverse water gas shift (RWGS) side reaction (CO2 + H2 = CO + H2O; ∆ H0(298 K) = 41.2 kJ/mol).13-15 In recent years, Han et al.16 and Mohanty et al.17 synthesized MAS-9, MTS-9 and mesoporous alumino-silicate (MAS-24) via the introduction of heteroatoms, Al, Ti into the framework of mesoporous silica, these materials showed higher hydrothermal stability, in comparison to those other pure silica materials. Specially, the wall of MAS-24 consisted of microporous ZSM-5 structure, which ensured the MAS-24 support possessed the characteristic of both micropore (zeolites) and mesopore. Moreover, the addition of alkaline earth metal oxides, for instance, the basic MgO, CaO, and BaO18, 19 in Ni-based catalysts can not only improve the activity of the catalysts but also inhibit the formation of coke. Recently, Wang et al.20 reported that the MgO-coated Ni/SBA-15 catalysts exhibited better catalytic activity and stability, and Son et al.21 claimed that the conversions (97%, 95%) of CO2 and CH4 over MgO promoted Co-Ni/Al2O3 catalyst were much higher than these (89%, 85%) over CoNi/Al2O3 under the reaction conditions (850 oC, 1 atm, GHSV = 4 L·gcat−1·h−1, CH4:CO2:N2 = 1:1:1). On the one hand, the formation of NiO-MgO solid solution makes the reduction of NiO become more difficult, inhibiting the sintering of metal Ni0 particles and leading to the higher dispersion and smaller particle size of nickel oxide. On the other hand, after introducing MgO, the basic sites of the catalyst increase, and the ability of the CO2 adsorption is improved naturally, which is

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beneficial to the elimination of carbon deposition by reverse disproportionation (CO2 + C = 2CO; ∆ H0 (298K) = 171 kJ/mol).22-24 According to the our previous reports, the results of CO2/CH4 reforming reaction over

La2NiO4/ZSM-5,25

LaxNiyOz/MAS-9

28

Ni/SL

(sol-gel),26

Ni/Sm2O3-CaO(1:4)27

and

catalysts revealed that approximately 10wt% Ni loadings would

exhibit excellent catalytic activity and stability, therefore, the amount of nickel loadings of 10wt% were employed hereinafter. In addition, even though NiO-MgO solid solution and Ni-supported mesoporous silica catalysts were reported partially, MgO-promoted 10Ni/MAS-24 catalysts with ZSM-5 structure for CH4/CO2 reforming reaction was still not reported yet. The introduction of aluminum atoms could effectively improve the hydrothermal stability of the support MAS-24 with stable ZSM-5 structure and the effect of the acidic sites29 on MAS-24 can be eliminated by adding the appropriate amount of magnesium oxide. Therefore, MAS-24 was synthesized from the assembly of ZSM-5 nano-clusters as a catalyst support and a series of 0~12wt%MgO-10Ni/MAS-24 catalysts were prepared by a sol-gel method for the CO2 reforming of methane.

2. EXPERIMENTAL 2.1. Catalyst Preparation. The mesoporous MAS-24 with ZSM-5 structure was synthesized according to the report of Mohanty et al.17 Firstly, the preparation of ZSM-5 nano-clustered solution included that 0.35 g of NaAlO2 (AR) was dissolved in 68 mL of deionized water (DW), followed by 14 mL of tetrapropylammonium hydroxide (TPAOH, 25%) and 24 mL of tetraethyl orthosilicate (TEOS, AR) under 4


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stirring (Al2O3/SiO2/Na2O/(TPA)2O/H2O molar ratios = 1.0/50/1.0/7/1800) at room temperature (RT). Then, the obtained clear solution was transferred into a Teflon-lined autoclave for hydrothermal treatment at 100 oC for 24 h. Secondly, after 10 g of EO20PO70EO20 (Sigma-Aldrich P123) was dissolved completely in 363 mL of HCl (2.7 mol/L), the obtained ZSM-5 nano-clustered solution was added drop-wise with constant stirring and kept at 40 oC for 24 h. Subsequently, the white mixture was moved to a Teflon-lined autoclave for additional reaction at 100 oC for 36 h. The obtained product was filtered and washed adequately with DW, dried and calcined in air at 550 oC for 5 h to obtain mesoporous MAS-24 via the removal of organic templates. A series of 0-12 wt%MgO-10wt%Ni/MAS-24 were prepared by a sol-gel method, i.e. the corresponding Ni(NO3)2·6H2O, Mg(NO3)2·6H2O and citric acid with a molar amount of 1.5 times that of the total metal ions were dissolved in 40 mL of DW, then the green liquor was mixed with an appropriate amount of as-prepared mesoporous MAS-24 and stirred at 60 oC until a stringy, transparent green gel was formed. The obtained gel was aged at RT for 3 d and dried at 120 oC with fast and uniform stirring. Finally, the light green powder was calcined at 600 oC (2 oC/min ramping rate) for 6 h to form the 0~12 wt%MgO-10wt%Ni/MAS-24 catalysts, which were denoted as 10Ni/MAS-24, 3MgO-10Ni/MAS-24, 6MgO-10Ni/MAS-24, 9MgO-10Ni/MAS-24 and 12MgO-10Ni/MAS-24, respectively, and all samples were pressed, crushed and sieved through 40-60 mesh. 2.2. XRD, BET, H2-TPR, CO2-TPD, UV, FT-IR, TG-DSC, HRTEM of

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Catalyst. The crystal structures of support and catalysts were characterized by wideand small-angle X-ray diffraction (XRD). The wide-angle XRD patterns were recorded on a PANalytical automatic diffractometer with Ni-filter Cu Kα radiation (λ = 0.15406 nm; operated at 40 kV and 50 mA) with a scanning rate of 10 oC/min. Small-angle XRD patterns were performed by a Rigaku D/max X-ray Diffractometer with a scanning rate of 1 oC/min. The N2 adsorption isotherms of all catalysts were measured using a home-made apparatus at 77 K (under the condition of liquid nitrogen). Catalyst (ca. 50 mg) was degassed at 200 oC in vacuum for 1 h and then, cooled to RT before measurements. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) in the relative pressure range p/p0 = 0.05-0.3 and pore size distribution via Barrett−Joyner −Halenda (BJH) method. H2 temperature-programmed reduction (H2-TPR) experiments were performed to study the reducibility of fresh catalysts. The fresh catalyst (ca. 50 mg) was put into a U-shaped quartz tube reactor and fixed by appropriate amount of asbestos. Before TPR experiment, samples were pretreated at 150 oC for 1 h in flowing of N2 (50 mL/min) in order to remove the surface water and any impurities. After the reactor was cooled down to the RT, a 5% H2/N2 (50 mL/min) mixture was introduced and the temperature was increased from RT to 900 oC at a rate of 10 oC/min. The signal of H2 consumption was recorded by a gas chromatography (103 G) with a thermal conductivity detector (TCD). CO2 temperature-programmed desorption (CO2-TPD) were employed to

6


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investigate the basicity of fresh catalysts. About 70 mg of fresh catalyst was firstly pretreated at 150 oC for 1 h in helium (30 mL/min), and then cooled down to RT, followed by the exposure of CO2 (20 mL/min) for 1 h. After helium purging for 0.5 h at RT (to remove the gas-phase and residual CO2 on the surface) to reach a stable baseline of CO2, the sample was heated from 25 oC to 900 oC with a ramp of 10 o

C/min in helium and the desorbed amount of CO2 was detected with the TCD. The UV-vis diffuse reflectance spectroscopy (UV-DRS) was performed over a

UV-2101 spectrometer. The species were scanned in the range of 200-800 nm by using BaSO4 as reference. Absorbance was converted to F(R) according to the Kubelk-Munk function (A = -lg(R), F(R) = (1-R)2/2R ). Fourier transformation infrared spectra (FT-IR) analysis was performed on a Nicolet Nexus 670 IR spectrometer with a resolution of 4 cm−1. Small amount of sample was mixed uniformly with dry KBr (ca. 1:100) and pressed into a transparent sheet, then fixed it on a dedicated metal plate and set up the wave-number range from 400 to 4000 cm−1. The FT-IR spectra of adsorbed pyridine were recorded by using an aforementined IR spectrometer. Before the pyridine adsorption, the samples degassed at 200 oC for 2 h under vacuum and then cooled to RT. Pyridine was then injected into the samples and maintained about 1 h, followed by degassed at 200 oC again in order to remove physisorbed pyridine. At last, the prepared samples were measured in the wave-number range of 1400-1600 cm−1. Thermo-gravimetric/differential scanning calorimetry (TG/DSC) experiments were carried out on a NETZSCH STA409PC/PG thermo-gravimetric analyzer to

7



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determine the amount and species of carbon deposited on the used catalysts. About 8 mg of used catalyst was heated from 35 oC to 900 oC at the atmosphere at a rate of 10 o

C/min. The high resolution transmission electron microscope (HRTEM) images of

support, fresh and used 9MgO-10Ni/MAS-24 catalysts were acquired on a Tecnai G2 F20 microscope operating at a voltage of 200 kV to investigate the morphology and structures of carbon deposition. The samples were prepared according to the report in literature.30 2.3 Catalytic Activity and Stability of Catalysts. The CO2 reforming of CH4 reaction was performed in a fixed-bed quartz reactor (8 mm i.d.) under atmospheric pressure (1 atm). Typically, 150 mg of fresh catalyst (40-60 mesh) was placed vertically to reactor in the center of a tubular furnace. The temperature (700 oC-800 oC) was controlled with a K-type of thermocouple located inside the reactor. Prior to the reaction, the catalyst was reduced in a flow of H2 (40 mL/min) at the corresponding reaction temperature with a heating rate of 10 oC/min. Then, CO2 and CH4 (1:1) were introduced into the catalytic bed via a mass flow meter and the gas hour space velocity (GHSV) was remained at 24 L·gcat−1·h−1. The effluent after removal of water (by an ice-water trap) was analyzed by an on-line 102 G gas chromatography with a TCD. The kinetics of the CH4/CO2 reforming reaction was described as follows: First, CH4 dehydrogenation generated H2 and carbonaceous CxH1

−

 5x − 1  xCH 4 → C x H 1− x +  H 2  2 

Secondly, CxH1

−

x

x

31

: (1)

interacted with carbon dioxide to produce synthesis gas:

8


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1− x  C x H 1− x + xCO2 → 2 xCO +  H 2  2 

(2)

The CO2 reforming of CH4 reaction (3) was the integration of equation (1) and (2): CH 4 + CO 2 → 2H 2 + 2CO; ∆H 0 = 247 kJ ⋅ mol −1

(3)

RWGS side reaction： H 2 + CO2 → H 2 O + CO; ∆H = 35kJ ⋅ mol −1

(4)

The conversion of CH4 (XCH4) and CO2 (XCO2), the selectivity of H2 (SH2) and CO (SCO), the carbon yields (YC) and H2/CO molar ratio were calculated by the following formulas:

X CH 4 =

( FCH 4 − FCH 4 in

FCH 4

out

FCO out ) + ( F CH 4 − FCH 4

( FCO2 − FCO2

S H2 =

2×( FCH 4 − FCH 4

YC =

× 100 % ;

in

out

in

out

in

out

)

R( H 2/ CO) =

out

)

( FCO2 − F CO in

FCO 2

2 out

in

out

+ FCO 2

( FCO 2 + FCH 4 ) in

)

× 100% ;

(6)

in

× 100 %;

(7)

× 100%

[( FCO 2 + FCH 4 ) − ( FCH 4 in

X CO 2 =

(5)

in

S CO =

FH 2

)

(8) out

+ FCO

out

)]

× 100 % ;

(9)

in

FH 2out

(10)

FCOout

Where, FCH4 in and FCO2 in represented the inlet flow rates (mL/min) of CH4 and CO2 (before reaction), respectively. FCH4 out, FCO2 out, FCO out, and FH2 out represented the outlet flow rates (mL/min) of CH4, CO2, CO and H2 (after reaction), respectively.

3. RESULTS AND DISCUSSION 3.1 XRD, BET, FT-IR Characterization of Samples. The small- and wide-angle XRD patterns of the as-prepared MAS-24 were shown in Figure 1A，it could be seen that there were two weak diffraction peaks at 2θ = 0.54o and 1.40o attributing to (100) and (110) plane diffraction,17 respectively, which exhibited the 9



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character of hexagonal mesoporous materials with p6mm symmetry structure. In addition, there were the characteristic peaks of ZSM-5 at 2θ = 8.06o, 8.9o, 14.9o, 20.9o, 23.1o, 29.8o, 45.5o (PDF 44-0003) to appear in the wide-angle XRD patterns, which indicated that the mesoporous wall of as-prepared MAS-24 composed of high stable ZSM-5 structure. The wide-angle XRD patterns (Figure 1B) of fresh and used catalysts illustrated that the diffraction peaks of NiO appeared at 2θ = 37.4o, 43.3o, 63.0o, 75.5o, 79.6o (PDF 73-1519) in fresh 10Ni/MAS-24, and after MgO was doped, the structure of Mg0.4Ni0.6O solid solution was detected at 2θ = 37.2o, 43.3o, 62.5o, 75.1o, 79.1o (PDF 34-0410) (Figure 1B(c)) while the diffraction peaks of NiO became weak. The size of NiO clusters (7.2 nm) on fresh 9MgO-10Ni/MAS-24 catalyst was significantly smaller than these (11.8 nm) over fresh 10Ni/MAS-24, which were calculated by Scherrer equation (D = Kλ/βcos θ) according to XRD patterns. This indicated that the doping of MgO promoted remarkably the high dispersion of NiO particles. Moreover, after the reduction of H2 and CH4/CO2 reforming reaction at 700 oC for 10 h, the NiO species in catalysts were reduced to metallic Ni0 [2θ = 45.5o, 51.8o, 76.3o (PDF 87-0712)]. We observed that the metallic Ni0 diffraction peaks over used 10Ni/MAS-24 were significantly higher than that over used 9MgO-10Ni/MAS-24 (7.5 nm) and the Ni0 particle size of the former (12.3 nm) was higher than that of the later catalysts, suggesting the sintering of metallic Ni particles was inhibited effectively in 9MgO-10Ni/MAS-24 catalyst. Meanwhile, the diffraction peak of graphic carbon was detected at 2θ = 26.0o (Figure 1B (b, d)). In addition, after the

10


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CH4/CO2 reforming reaction of ca. 60 h at 800 oC, the diffraction peaks of metallic Ni0 in 9MgO-10Ni/MAS-24 intensified and the occurrence of no graphic carbon diffraction peak, indicating that the aggregation of metal Ni0 might be the main reason causing the decrease of catalyst activity, which would be discussed hereinafter. ( 100)

•NiO ♦Ni Mg0.4Ni0.6O MgO C ZSM-5 (B)

ZSM-5 (A)

Intensity

e

d Intensity

20

♦

40

60

c

 







8

a 10 10

♦

♦

( 020) ( 101)

6

♦

( 110)

4

 ♦

80

b

2

♦

♦

2-Theta(deg)

0

♦

Intensity

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20

30

♦

• • 40

50

• 60

•• 70

80

2-Theta(deg)

Figure 1. (A) Small-angle XRD pattern of MAS-24 (inset is wide-angle XRD of MAS-24); (B) wide-angle XRD patterns of (a, b) fresh and used 10Ni/MAS-24 for 10 h, (c, d) fresh and used 9MgO-10Ni/MAS-24 for 10 h, and (e) used 9MgO-10Ni/MAS-24 for 60 h.

The N2 adsorption isotherms and pore size distributions of support, fresh and used catalysts were exhibited in Figure 2. It could be seen that all isotherms presented capillary coagulation which could be classified as IV-type curves, indicating that the mesoporous structure remained in all catalysts,21 the corresponding pore size distributions revealed in the range of 2-3 nm (Figure 2B). Meanwhile, the textural properties of support, fresh and used catalysts were listed in Table 1. With the loadings of NiO and MgO, the SBET, VT, Da of the corresponding catalysts suffered from the decline to a large extent due to the occupation of mesoporous channels by metal oxide particles and the central atom density (CAD) increased gradually (Table

11



1). However, it is interest to note that the variation trend in SBET of catalysts become smooth after 9MgO-10Ni/MAS-24 catalyst. The CAD of MgO or NiO/nm2 support estimated was approximately 10.3 for monolayer coverage according to the closely accumulation of NiO (Ni2+ ionic radius = 0.069 nm) and MgO (Mg2+ ionic radius = 0.072 nm, O2- ionic radius = 0.14 nm), similar to the CAD (11.8/nm2) over 9MgO-10Ni/MAS-24 catalyst. However, the CAD (13.8/nm2) over 12MgO10Ni/MAS-24 was significantly higher than the value (10.3/nm2) of atomic monolayer arrangement. Therefore, the SBET of 12MgO-10Ni/MAS-24 is slight higher than that of 9MgO-10Ni/MAS-24 due to multilayer stack up of excess MgO.

(A)

(B)

a b

Adsorption Volume (mL/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

c d e f g h i

0.0

0.2

0.4

0.6

0.8

Relative Pressure (p/p0)

1.0

10

100

Pore Diameter (angstrom)

Figure 2. (A) N2 adsorption isotherms (B) pore distribution of (a) MAS-24, (b, c) fresh and used 10Ni/MAS-24 for 10 h, (d) fresh 3MgO-10Ni/MAS-24, (e) fresh 6MgO-10Ni/MAS-24, (f, g, h) fresh, used 9MgO-10Ni/MAS-24 for 10 h and 60 h, (i) fresh 12MgO-10Ni/MAS-24.

In addition, after CH4/CO2 reforming reaction, the SBET or VT of all used catalysts only reduced slightly due to high thermal and hydrothermal stability of MAS-24 support. In the meantime, the variation in SBET of 9MgO-10Ni/MAS-24 (from 201 to 194 m2/g) before and after reaction was smaller than that of 10Ni/MAS-24 catalyst 12


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(from 357 to 298 m2/g) due to the synergistic action between MgO and Ni0, and after CH4/CO2 reforming reaction at 800 oC for 60 h, the physical properties (SBET, VT and Vmeso) of 9MgO-10Ni/MAS-24 declined very slight.

Table 1. Specific Surface Area (SBET), Total Pore Volume (Vt), Average Pore Diameter (Da), Central Atom Density (CAD), Mesoporous- (Vmeso) and Micoporous- (Vmic) Volume of Support, Fresh and Used Catalysts. CAD SBETa

Samples

2

(m /g)

VT b 3

(cm /g)

Vmic b 3

(cm /g)

Vmeso b 3

(cm /g)

Da c (nm)

(MeOX/nm2) NiO/Ni0

MgO

MAS-24

462

0.548

0.167

0.381

4.75

--

--

10Ni/MAS-24

357

0.390

0.115

0.275

4.37

2.87

--

3MgO-10Ni/MAS-24

286

0.323

0.086

0.237

4.52

3.58

1.58

6MgO-10Ni/MAS-24

266

0.293

0.087

0.206

4.41

3.85

3.40

9MgO-10Ni/MAS-24

201

0.238

0.065

0.173

4.73

5.10

6.70

206

0.222

0.070

0.152

4.31

5.00

8.80

12MgO-10Ni/MAS-24 d

298

0.310

0.098

0.212

4.16

3.44

--

Used 3MgO-10Ni/MAS-24(10 h)

d

252

0.280

0.085

0.194

4.44

4.06

1.79


d

247

0.275

0.071

0.204

4.45

4.15

3.65


d

194

0.228

0.065

0.163

4.70

5.28

6.98

Used 10Ni/MAS-24(10h)

Used12MgO-10Ni/MAS-24(10 h)

d

Used 9MgO-10Ni/MAS-24(60 h)e

173

0.181

0.065

0.116

4.16

5.92

10.4

170

0.180

0.065

0.115

4.23

6.03

8.00

a

SBET, specific surface areas were estimated from N2 adsorption at 77 K, b VT (total pore volume) and Vmic (micropore volume) were obtained at p/p0 = 0.95 and 0.1, respectively. Vmeso (mesopore volume) was calculated via the subtraction of Vmic from VT. c Da, average pore diameters were calculated by formula: 4VT ×103/SBET. d The catalysts tested under the reaction conditions: CH4/CO2 = 1,GHSV = 24 L·gcat−1·h−1, 700 oC, 1 atm, e Stability test under the reaction conditions of CH4/CO2=1, GHSV=24 L·gcat−1·h−1, 800 oC, 1 atm.

As shown in Figure 3A, The FT-IR spectra of support MAS-24, fresh 10Ni/MAS-24 and 9MgO-10Ni/MAS-24 catalysts illustrated that the band at 3650 cm-1 was assigned to hydroxyl groups associated with the extra framework of aluminum species, and the broad band at ca. 3425 cm-1 originated from the O-H stretching vibration of adsorbed water.28 The weak bands at 1627 and 963 cm-1

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

corresponded to the bending and stretching vibrations of Si-OH groups, respectively. Meanwhile, the bands at 1085 cm-1, 800 cm-1 and ca. 460 cm-1 were attributed to the asymmetrical and symmetrical stretching vibration as well as bending vibration of Si-O-Si. The band at 550 cm-1 verified the presence of the five-membered ring of Si-O-Al with ZSM-5 structure in mesoporous MAS-24.32 However, the band at around 3425 cm-1 in 9MgO-10Ni/MAS-24 became bigger than that in MAS-24 and 10Ni/MAS-24 due to the fact that the addition of MgO would lead to the absorption of more water. Meanwhile, the band at 1627 cm-1 in intensity enhanced, meaning the formation of carbon species related with CO2 adsorption on MgO21,33 or the O-H bending vibrating mode of the adsorbed interlayer water molecules mentioned above.34 In addition, the band at 963 cm-1 in 9MgO-10Ni/MAS-24 and 10Ni/MAS-24 catalysts disappeared because of the interaction between metal ions and the H+ on Si-OH bonds. The MAS-24 structure remained intact after the addition of NiO and MgO on the basis of the corresponding vibration groups of framework. The Py-FTIR spectra of the samples in the region of 1400-1600 cm-1 were shown in Figure 3B. According to the report in literature,17,35 the two bands at 1538 cm-1 and 1445 cm-1 were assigned to Bronsted and Lewis acid sites, respectively, and the band at 1495 cm-1 derived from pyridine co-adsorbed on both Bronsted and Lewis acid sites. As being observed in Figure 3B, the MAS-24 displayed three bands after degassing at 200 oC, indicating that MAS-24 performed a certain amount of acid strength due to the aluminum incorporation into the framework of Si-O-Si. Besides, the intensity of bands weakened obviously with the loading of metal oxide, especially for

14


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Page 15 of 39

9MgO-10Ni/MAS-24 catalyst due to the interaction between metals oxide and support or the addition of basic MgO, leading to the reduction of the acid sites.

1489 1538

Absorbance

(b )

4 000 350 0 3 000 250 0 2000 15 00 1000

(a )

(b )

(c)

800 550

L e w is 460

1085

960

1627

3650

(c)

3425

1445

(B )

(a) 963

(A )

Transmittance(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 00

B ro n tste d

B ro n tste d + Le w is

1580 15 60 1 540 1 520 150 0 14 80 1 460 144 0 -1

W a v e nu m b e r(cm )

Figure 3. FT-IR spectra (A) before and (B) after pyridine adsorbed over (a) MAS-24, (b) fresh 10Ni/MAS-24, (c) fresh 9MgO-10Ni/MAS-24.

3.2 H2-TPR and CO2-TPD Spectra of MAS-24 and Catalysts. As shown in Figure 4A, the reduction behavior of NiO in catalysts depended strongly on the MgO loadings. There were a major peak at 400 oC, assigned to the reduction of bulk NiO species via the weak interaction with MAS-24, and two shoulder peaks at 523 oC and 684 oC in TPR profiles of fresh 10Ni/MAS-24, which were ascribed to the reduction of Si-O-Ni or Al-O-Ni on Bronsted or Lewis acid sites of MAS-24. For MgO-doped 10Ni/MAS-24 catalysts, the reduction peaks of NiO particles shifted towards high or low temperature (Figure 4A(b-e)) due to the interaction of metal and MAS-24 as well as the high dispersion of nickel species caused by the segregation effect of nano-MgO particles.9,20 The peaks under 600 oC could be attributed to the reduction of NiO on the surface or in the bulk of MgO structure

36,37

while the addition of MgO had a

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

promoting effect for the reduction of highly dispersed NiO species.38 However, the peaks at 728 oC and 739 oC (Figure 4A(d,e)) correlated closely with the reduction of Ni2+ ions located in the subsurface layers of the MgO lattice, showing a stronger interaction between NiO and MgO. The CADs over 9MgO-10Ni/MAS-24 (11.8 /nm2) and 12MgO-10Ni/MAS-24 (13.8 /nm2) also were significantly higher than that (10.3 /nm2) of atomic monolayer coverage (Table 1). Therefore, the more reduction sites at high temperature presented over 9MgO-10Ni/MAS-24 catalyst (Figure 4A(d)) was due to the most reasonable MgO/NiO molar ratio, which was more stable and uniform. As described in Figure 4B, the CO2-TPD profiles showed that there was a strong desorption peaks of CO2 at 125 oC for all MgO-doped catalysts and the same situation occurred in 10Ni/MAS-24 catalyst, which might be stemmed from both the weakly chemisorbed CO2 of the framework and the physically adsorption of CO2 because of the large specific surface areas as well as pore volumes owned by these catalysts.39 In addition, another desorption peak at 390 oC for 10Ni/MAS-24 catalyst was also observed, it could be assigned to the weakly chemical adsorption of CO2 (Figure 4B(a)). Nevertheless, there were a successive and broad desorption peak of CO2 over MgO-doped 10Ni/MAS-24 catalysts (Figure 4B(b-e)) when temperature was more than 200 oC, especially at around 856 oC, which was related to strong chemisorption of CO2 over the moderate and strong basic sites on the surface of catalyst.39,40 The increase of basic sites was also in line with the result of Py-FT-IR spectra (Figure 3B) concerning the reduction of the acid sites. Meanwhile, all peaks in intensity increased

16


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Page 17 of 39

with the addition of MgO, especially for 9MgO-10Ni/MAS-24 catalyst. In other words, the doping of MgO improved significantly the basic sites of catalyst due to the inherent property of MgO.41 Generally, the enhancement of the basicity was more favorable for the adsorption of acidic CO2 and can suppress effectively the formation of coke by supplying the surface oxygen species.42

(A)

530

(B)

125

372 739

390

728

(e)

(e)

856

523

TCD Signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(d) (d)

452 413

(c)

660

(c) 534

786

400 696

(b)

(b)

523 684

(a)

(a) 200

400

600

800

0

200

400

600

800

Temperature (oC)

Figure 4. (A) H2-TPR (50 mg) and (B) CO2-TPD profiles (70 mg) over fresh (a) 10Ni/MAS-24, (b) 3MgO-10Ni/MAS-24, (c) 6MgO-10Ni/MAS-24, (d) 9MgO-10Ni/MAS-24, (e) 12MgO-10Ni/ MAS-24.

3.3 UV-vis Diffuse Reflectance Spectroscopy of Catalysts. The UV-DRS of fresh and used 0-12wt%MgO-10Ni/MAS-24 catalysts reformed at 700 oC for 10 h were shown in Figure 5. For all catalysts, there were a big peak at 250 nm and a shoulder peak at around 300 nm, which was typically associated to O2-(2p) →Ni2+ (3d) charge transfer transitions and revealed the presence of coordinated tetrahedrally Ni species on the framework of MAS-24 (Figure 5A). In the meantime, a weak peak at around 405 nm appeared and intensified with incremental MgO loadings. According to the report of Gao et al,43 the band at about 405 nm was assigned to octahedral Ni species 17



or NiO generated on the surface of the support, which was associated to d-d transitions of Ni2+. In other words, tetrahedrally coordinated Ni species was dominated in fresh 0~12%MgO-10%Ni/MS-24 catalysts. The shape and position of the absorption band in the UV region correlated with the shape and size of NiO particles. The absorption bands edge of DRS for all MgO-doped catalysts shifted towards lower wavelength compared with the 10Ni/MAS-24 catalyst.44

(B)

(A) (e) (d) (a) (b)

F(R)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(c)

(c) (b) (a)

250

(d) 300 405

(e)

200 300 400 500 600 700 800 200 300 400 500 600 700 800 Wavelength( nm)

Figure 5. UV-vis DRS spectra of (A) fresh and (B) used (a)10Ni/MAS-24, (b) 3MgO-10Ni/MAS-24, (c) 6MgO-10Ni/MAS-24, (d) 9MgO-10Ni/MAS-24, (e) 12MgO-10Ni/ MAS-24.

This was mainly reflected in the obvious weakening in intensity of the peak at 300 nm and the corresponding enhancement in intensity of the peak at 250 nm with the increasing MgO loadings, which was a phenomenon of blue shift,45 implying that the bandgap energy increased after the addition of MgO and further led to the decrease in size of NiO particles, in accordance with the XRD results (Figure 1B). As for the used catalysts, the absorption bands of 250 nm almost disappeared and then presented a 18


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continuous absorption in the entire region (Figure 5B), which was most likely due to the reduction of NiO to the metallic nickel.46 3.4 TG-DSC Analysis. The properties of carbon depositions over used 10Ni/MAS-24 and 9MgO-10Ni/MAS-24 catalysts were investigated by TG/DSC technique and the results were shown in Figure 6. There was a weight loss over used catalysts at lower than 200 oC in the TG curves due to moisture evaporation. Then, there was a slight enhancement in weight in the range of 250-500 oC,47 which originated plausibly from the overlapping results of the removal of active carbon species (C + O2 = CO2 or CO) and metallic Ni0 oxidation (2Ni0 + O2 = 2NiO), and a corresponding exothermic peak appeared at around 290 oC in DSC curves (Figure 6B). It is interest to note that the weight loss (5.9%) over used 9MgO-10Ni/MAS-24 was higher than that (2.0%) over used 10Ni/MAS-24, plausible due to the high catalytic activity of the former for CH4 decomposition.48 However, the weight loss over 9MgO-10Ni/MAS-24 used at 800 oC for 60 h was very small because the reaction of CO2 with coke occurred at high temperature. Besides, there were two distinct exothermic peaks at around 290 oC and 600 oC in the DSC profiles of used catalysts, implying that at least two kinds of carbon deposition produced on the surface of catalysts (Figure 6B). The peaks around 290 oC belonged to the combustion of active or hydrogen-containing carbon and the big peaks around 600 oC were assigned to amorphous or whisker-type carbon (nano-tube), which had a close relationship with the deactivation of the catalyst. Moreover, the exothermic peak over used 9MgO-10Ni/MAS-24 shifted towards lower temperature compared to 10Ni/MAS-24,

19



indicating that the more insert carbon species are generated over 10Ni/MAS-24 catalyst. (A)

(c)

100

7

(B)

6

exothermic

102

5

98

(a) 96

DSC(mw/mg)

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

629 600 (a)

4

(b) 294

3

293

2

94

280

1 92

10Ni/MAS-24(700,10h) 9MgO-10Ni/MAS-24(700,10h) 9MgO-10Ni/MAS-24(800,60h)

200

400

600

(b)

0

200

800

(c)

400

600

800

Temperature (oC)

Figure 6. (A) TG/ (B) DSC profiles of (a) 10Ni/MAS-24, (b) 9MgO-10Ni/MAS-24 used at 700 oC for 10 h, and (c) 9MgO-10Ni/MAS-24 used at 800 oC for 60 h.

3.5 HRTEM Analysis. The HRTEM images of MAS-24, fresh and used 9MgO-10Ni/MAS-24 at 800 oC for 60 h were shown in Figure 7, it can be seen that MAS-24 presented disordered mesoporous arrays with three-dimensional structure (Figure 7A), as displayed in the corresponding magnification images (Figure 7B-C), similar to the mesoporous structure of MCF49,50 with uniform spherical pores interconnected by window. Moreover, the selected area election diffraction (SAED) pattern of MAS-24 exhibited in the arrays of regular lattice dots and the result calculated by Fourier transformation of distance between two light dots revealed that the interplanar d-spacing was about 0.38 nm, corresponding to the (-5, 0, 1) plane of ZSM-5 crystal structure (Figure 1B), which indicated that mesoporous wall of MAS-24 consisted of the high stable ZSM-5 crystallites (Figure 7D). In addition, SAED patterns in a large region presented a highly dispersed circle (Figure 7E), this

20


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suggested the disordered structure of MAS-24 in long ranges. After MgO and NiO nanoparticles were loaded by a sol-gel process, MAS-24 structure almost remained intact (Figure 7F), and the corresponding to SAED pattern (Figure 7G) revealed high dispersion rings with disordered light dots, indicating that MgO and NiO was highly dispersed on the channel of MAS-24. As shown in Figure 7G, the interplanar distances of the first (d1 = 0.21 nm) and second (d2 = 0.15 nm) spotty rings were in consist with the interplanar d-spacing of (200) and (220) crystal plane of Mg0.4Ni0.6O species, respectively (Figure 1B (c)) based on Bragg equation (nλ = 2dsinθ). The result of EDX analysis (Table 2) demonstrated that the ratio of Mg and Ni (Mg wt% = 8.95%, Ni wt% = 13.44%) over fresh catalyst was very closely to the stoichiometric value of initial 9MgO-10Ni/MAS-24 catalyst. In other words, the MgO were well doped on 10Ni/MAS-24 catalyst. After CH4/CO2 reforming reaction at 800 oC for 60 h, the Ni0 particles aggregated slightly (ca.14-25 nm) (Figure 7I, L), meanwhile, the SAED pattern (Figure 7J) of used 9MgO-10Ni/MAS-24 catalyst presented high clear rings with a large amount of scattered bright spots. Besides, the result of EDX analysis (Figure 7K) for the black spots over used catalyst indicated that the aggregation of Ni0 particles was remarkable. However, there is no carbon deposition over used 9MgO-10Ni/MAS-24 based on the HRTEM images.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

E

Page 22 of 39

D

A SAED

SAED

C

HRTEM

B F

(220)

G d 2=0.15

(200) d1=0.21

Ni

SAED

H EDX HRTEM

J

K EDX Ni

SAED

HRTEM Ni

L

I

Figure 7. HRTEM images (A, B, C, F, I, L), EDX analysis (H, K) and SAED images (E, D, G, J) of support MAS-24(A-E), fresh 9MgO-10Ni/MAS-24 (F-H) and used 9MgO-10Ni/MAS-24 at 800 oC for 60 h (I-L).

22


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. EDX Analysis: Elemental Composition of Fresh and Used 9MgO-10Ni/MAS-24 at 800 oC for 60 h

Elements O Mg Si Ni

fresh

used

Wt %

Atomic %

Wt %

Atomic %

33.61 8.95 43.61 13.44

49.31 8.65 36.45 5.36

8.33 1.27 7.02 67.4

15.75 1.41 7.56 34.7

3.6 Catalytic Activity and Stability of Catalysts. 3.6.1 Effect of Different Magnesium Oxide Loadings. The effects of different MgO loadings over 0~12wt%MgO-10Ni/MAS-24 catalysts on the catalytic performances were investigated by reforming reaction under the conditions of CH4/CO2 = 1, 700 oC, 1 atm for 10 h. As shown in Figure 8, the conversion of CH4 (from 65% to 43%) and CO2 (from 85% to 72%) over 10Ni/MAS-24 reduced gradually whereas the corresponding results over all MgO-doped 10Ni/MAS-24 catalysts almost remained 72% and 90% conversions of CH4 and CO2 with incremental time on stream (Figure 8) and were remarkably higher than these over 10Ni/MAS-24 due to the fact that the relative strong acidity of MAS-24 without modification of MgO would reduce the adsorption of CO2 (confirmed by CO2-TPD analysis) despite of the hydrothermal stability of MAS-24 itself . FT-IR spectra of MAS-24 also confirmed the existence of acidic sites, similar to report of Sarker et al.51 On the other hand, the large size of active particles over 10Ni/MAS-24 will result in the decline of the catalytic activity and stability in CH4/CO2 reforming reaction. In other words, the addition of MgO would effectively weaken the surface acidity of support and induce the formation of Mg0.4Ni0.6O solid

23



solution to improve the dispersion of NiO particles. XRD results verified that the particle size of Ni (7.5 nm) over 9MgO-10Ni/MAS-24 was much smaller than that (12.3 nm) over 10Ni/MAS-24 after reforming reaction. Consequently, MgO-doped 10Ni/MAS-24 catalysts presented high stable catalytic activities. 82

(A)

(B)

93

80

(C)

0.91 0.90

92

78

0.89 91

76

0.88

CO2 conversion( %)

CH4 conversion( %)

74

90

72

0.87

H2/CO Ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 39

0.86

89 85

60 55

0.80

80

50 75 10Ni/M AS-24 3MgO-10Ni/MAS-24 6MgO-10Ni/MAS-24 9MgO-10Ni/MAS-24 12MgO-10Ni/MAS-24

45 40 0

2

4

6

0.75

70 8

10

0

2

4

6

8

10

0

2

4

6

8

10

Time on stream (h)

Figure 8. Conversion of CH4 (A), CO2 (B), and H2/CO molar ratio (C) observed over 0~12MgO-10Ni/MAS-24; GHSV = 24 L·g-1·h-1, CO2/CH4 = 1:1, T = 700 oC.

In addition, the CH4 and CO2 conversion over 0-12wt%MgO-10Ni/MAS-24 increased and then decreased with incremental MgO loadings. On 9MgO-10Ni/ MAS-24 catalyst achieved the maximum with CH4 and CO2 conversion of 79% and 92.5%, respectively. The addition of MgO would increase the basic sites on the surface of catalysts and induce the chemisorption of CO2, which was favorable for CH4/CO2 reforming reaction and CO2-TPD spectra (Figure 4B) also verified that the desorption peaks of CO2 at high temperature intensified with MgO loadings. However, the excessive MgO would cause a blockage of pore channels of MAS-24 and cover

24


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partial nickel active sites to decrease the activity of the catalyst.47 According to calculated CAD on different catalysts (Table 1), the distribution of MgO particles (8.80/nm2) on 12MgO-10Ni/MAS-24 was higher than that (6.74/nm2) on 9MgO10Ni/MAS-24 with similar density (5.0~5.1/nm2) of NiO particles, which achieved maximum (5.1/nm2) of Ni active sites for monolayer coverage and there were more proper distribution of MgO particles on the latter. Wang et al

52

also reported the

similar result over other catalysts, the relationship of the catalytic activity and stability with MgO loadings is Ni/8MgO-SBA-15 > Ni/15MgO-SBA-15 > Ni/SBA-15. It was noticeable that the H2/CO ratio over 9MgO-10Ni/MAS-24 catalyst (about 0.89) was higher than that over 10Ni/MAS-24 (0.86) (Figure 8C), indicating the RWGS reaction (CO2 + H2 = CO + H2O; ∆ H0 (298 K) = 41.2 kJ/mol) over the former reduced remarkably with the doping of MgO. The water collected by downstream ice-trap confirmed the appearance of the RWGS reaction.53 Besides, the RWGS side reaction also was the reason that why the conversion of CO2 was significantly higher than that of CH4. 3.6.2 Effect of Reaction Temperatures. Figure 9 showed the conversion of CH4 and CO2, selectivity of H2 and CO as well as carbon yield over 9MgO-10Ni/MAS-24 catalyst at 700-800 oC, it could be seen that the conversion of CH4 and CO2 increased gradually with incremental reaction temperature owing to the endothermic characteristic of CH4/CO2 reforming reaction (CH4 + CO2 = 2H2 + 2CO; ∆H0 (298 K) = 247.3 kJ/mol).54 With the temperature rise from 700 oC to 800 oC, the conversion of CH4 and CO2 increased

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

from 79% and 92.5% to 96% and 99%, respectively, and kept a constant at same temperature within 10 h, which were higher than these (94.6%, 96%) over M-15Ni2Mg83Al catalyst

39

(with the reaction conditions of CH4/CO2 = 1, 800 oC,

GHSV = 15 L·gcat−1·h−1) due to the utilization of high stable MAS-24 support with structure of ZSM-5. Before and after CH4/CO2 reforming reaction, the SBET of 9MgO-10Ni/MAS-24 (from 201 to 194 m2/g) almost kept a constant (Table 1). In addition, the carbon deposition at 800 oC (ca. 0.1%) was much lower than that at 700 o

C (ca. 0.7%) or 750 oC (ca. 0.35%) over 9MgO-10Ni/MAS-24 due to the fact that it

was more favorable at 800 oC for the reverse Boudouard reaction (CO2 + C = 2CO; ∆H0 (298 K) = 171 kJ/mol)28 or the reaction of carbon gasification (H2O + C = CO + H2; ∆H0 (298 K) = 131 kJ/mol). According to the reports in literature,55 the formation of carbon deposition over nickel-based catalyst was attributed to the catalytic cracking of methane (CH4 = 2H2 + C; ∆ H0 (298 K) = 75 kJ/mol) and the Boudouard reaction (2CO = CO2 + C; ∆H0 (298 K) = −171 kJ/mol), and it was mainly responsible for the deactivation of the catalyst. The high activity and stability of 9MgO-10Ni/MAS-24 catalyst mentioned above namely derived from low carbon yield (below 1%) in the range of 700-800 oC. Moreover, as shown in Figure 9, the effect of temperature on CH4 conversion was significantly higher than that on CO2 conversion due to the fact that the Gibbs free energy (G < 0) of CH4 dry reforming and CH4 dehydrogenation decreased gradually with increasing reaction temperature. Siahvashi et al.56 also reported the thermodynamic results of propane dry reforming.

26


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Page 27 of 39

100 95

Conversion, selectivity and carbon yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

100

95

95

90

90

90 85

85

85

80

Convension of CH4

(A)

75

80

Convension of CO2 Selectivity of CO Selectivity of H2

(B)

(C)

80

Carbon yield

1.0

1.0

1.0

0.5

0.5

0.5

0.0 0

2

4

6

8

0.0 0 10

2

4

6

8

0.0 10 0

2

4

6

8

10

Time on stream (h) Figure 9. Conversion of CH4 and CO2, selectivity of CO and H2, as well as carbon yield over 9MgO-10Ni/MAS-24 catalyst during CO2/CH4 reforming at (A) 700 oC, (B) 750 oC, (C) 800 oC; GHSV = 24 L·g-1·h-1; CO2/CH4 = 1:1.

Figure 10 showed the H2/CO ratios at different temperatures, which is less than 1 due to the RWGS reaction (CO2 + H2 = CO + H2O, ∆H0 (298 K) = 41 kJ/mol). However, the H2/CO ratio increased from 0.89 to 0.96 when the reaction temperature ramped from 700 oC to 800 oC, indicating the consumption of H2 via the RWGS reaction was inhibited to a certain extent with increasing temperature (i.e. the amount of water detected in the cold-trap declined from 1.0 g to 0.41 g). Meanwhile, the high temperature would favor the occurrence of catalytic cracking of methane (CH4 = 2H2 + C; ∆H0 (298 K) = 75 kJ/mol) due to its endothermic characteristics, which led to an increase of H2 production and coke formation, subsequently, the coke was removed by the reverse Boudouard reaction (CO2 + C = 2CO; ΔH0 (298 K) = 171 kJ/mol). Therefore, there was not significantly coke formation to be observed on the surface of catalyst due to aforementioned coupling reaction. In a word, it could be possible that 27



an increase in reaction temperature was more suits DRM (CH4 + CO2 = 2H2 + 2CO; ∆H0 (298 K) = 247.3 kJ/mol)54 than RWGS reaction, resulting in the increase of H2/CO ratios.

0.975

0.950

R (H2/CO)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.925

0.900

o

700 C o 750 C o 800 C

0.875

0.850 0

2

4

6

8

10

Time on stream (h)

Figure 10. H2/CO ratios over 9MgO-10Ni/MAS-24 catalyst at different temperatures, reaction conditions at GHSV = 24 L·g-1·h-1, CO2/CH4 = 1:1, 1 atm.

3.6.3 Catalyst Stability and Deactivation Analysis. The long term stability of catalysts was crucial in CO2/CH4 reforming reaction. According to the report in literature,36,47 the investigations in stability for MgO-doped Ni-based catalyst or NiO-MgO solid solution catalyst were performed under relatively lower temperature. Here, the stability of 9MgO-10Ni/MAS-24 catalyst was investigated at 700 oC for 72 h and 800 oC for 60 h, respectively, because of high activity and stability of MAS-24 whereas mesoporous MCM-41 and SBA-15 exhibited relatively poor hydrothermal stability as reported in literature.35 As shown in Figure 11A, during CO2/CH4 reforming reaction at 700 oC for 72 h, the conversion of 28


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CH4 maintained at around 78% and that of CO2 was 92% within 45 h, which is similar to CH4 conversion (approximately 78%) and is higher than the conversion of CO2 (82%) over NiO/MgO-Al2O3 at 700 oC reported by Alipoura.24 Subsequently, the activity of 9MgO-10Ni/MAS-24 catalyst decreased gradually. Meanwhile, there was only a slight decrease of the catalytic activity over 9MgO-10Ni/MAS-24 catalyst within the first 30 h at 800 oC (Figure 11B), the conversion of CH4 and CO2 was above 93% and 99%, respectively. In the later 30 h, the CH4 conversion decreased from 93% to 83% because the activity of 9MgO-10Ni/MAS-24 catalyst reduced with incremental time on stream and the conversion of CO2 still remained ca. 99% for 60 h due to the occurrence of RWGS reaction. According to the report of Zanganeh et al,57 the Ni0.1Mg0.9O catalyst calcined at 600 oC showed the highest conversions of methane

(72%)

and

CO2

(80%)

at

700

GHSV = 1.4 ×104 mL/h·gcat. Moreover, Min et al

o

C

58

with

CH4/CO2 = 1:1

and

reported that deactivation rate

(between 3 h and 37 h) of Ni-MgO-Al2O3 [MgO/(MgO+Al2O3) = 0.44] for CH4 conversion at 800 oC was 3.86%, higher than that (3.15%) of 9MgO-10Ni/MAS-24. In addition, the selectivity of H2 and CO over 9MgO-10Ni/MAS-24 were close to 100% at 800 oC for 60 h because of RWGS reaction (CO2 + H2 = CO + H2O, ∆H0 (298 K) = 41 kJ/mol), carbon gasification (H2O + C = CO + H2; ΔH0 (298 K) = 131 kJ/mol) and the reverse Boudouard reaction (CO2 + C = 2CO; ΔH0 (298 K) = 171 kJ/mol) to maintain their conversion close to 99%.51,59 Meanwhile, it was noteworthy that carbon yield over 9MgO-10Ni/MAS-24 was approximately 1% (700 oC) and 0.2% (800 oC) in a long term reforming reaction, the

29



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low carbon yield stemmed from the enhancement of Lewis basic sites (MgO) for adsorption of acidic CO2. The H2-TPR results revealed that the Ni active species in 9MgO-10Ni/MAS-24 can be reduced completely at 800 oC and Ni2+ existed as metallic Ni0. The catalysts was impacted by several deactivation factors, such as carbon deposition, the sintering of active metal particles and oxidation of metallic phase.51 TG/DSC results indicated that the weight loss over spent 9MgO-10Ni/MAS-24 at 800 o

C was very small (Figure 6A) and the corresponding exothermic peak (Figure 6B)

around 600 oC reduced remarkably in intensity, implying the absence of coke on the surface of catalyst. Moreover, the active Ni0 particles became bigger (14-25 nm) over used catalyst after CH4/CO2 reforming at 800 oC for 60 h compared to fresh catalyst (7.2 nm) based on XRD patterns (Figure 1B) and HRTEM images (Figure 7). Therefore, the aggregation of Ni0 particles was the main reason leading to the decrease of catalytic activity in the later 30 h. TG/DSC and HRTEM results for spent 9MgO-10Ni/MAS-24 catalyst jointly revealed that there was almost no coke formation on the surface of used 9MgO-10Ni/MAS-24 catalyst for 60 h. Therefore, 9MgO-10Ni/MAS-24 catalyst is high promising for industrialization in the near future.

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Conversion, selectivity and carbon yield (%)

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100

100

90

90

(A)

80

(B)

80

70

70

60

60

2

2

Conversion of CH 4 Conversion of CO 2 Selectivity of CO Selectivity of H2

1

1

Carbon yield

0

0 0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

Time on stream (h)

Figure 11. Long-term stability test over 9MgO-10Ni/MAS-24 catalyst at (A) 700 oC for 72 h and (B) 800 oC for 60 h. GHSV = 24 L·g-1·h-1; CO2/CH4 = 1:1; 1 atm.

4. CONCLUSIONS MAS-24 with strong acidity, large specific surface areas and high hydrothermal stability was synthesized successfully, in the meantime, a series of 0~12wt% MgO-10Ni/MAS-24 possessed high surface areas were prepared by a sol-gel method and employed in CH4/CO2 reforming. The addition of moderate MgO could improve effectively the catalytic activities of 10Ni/MAS-24. Among all MgO-doped catalysts, the 9MgO-10Ni/MAS-24 presented the highest catalytic activity at 700 oC for 10 h during CH4/CO2 reforming reaction. The conversion of CH4 and CO2 almost kept at ca. 79% and 92.5%, respectively. The catalytic activity of 9MgO-10Ni/MAS-24 almost maintained constant for 45 h with the conversion of CH4 (78%) and CO2 (92%). Although the conversion of CH4 decreased from 96% to 83% during the stability test of 60 h at 800 oC, it maintained at high level (from 96% to 93%) within 31



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30 h. The results of TG/DSC and HRTEM verified that the 9MgO-10Ni/MAS-24 catalyst possessed strong ability of anti-coke. Therefore, 9MgO-10Ni/MAS-24 catalyst with high hydrothermal stability and activity is a promising for the industrialization of CH4/CO2 reforming reaction.

■ AUTHOR INFORMATION Corresponding Author Tel.: 86-22-27892471. E-mail: [email protected].

■ Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was financially supported by Nation Natural Science Foundation of China and BAOSTEEL Group Corporation (grant no. 50876122).

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For Table of Contents Only Ni

MgO

Mg0.4Ni0.6O

CH4+CO2 CO+H2

MAS-24 10Ni/MAS-24

CO+H2

MAS-24 MgO-doped 10Ni/MAS-24

39


MAS-24 with Microporous

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